In bone surgery, the problem of prosthesis bio-compatibility has only been partially solved heretofore, since although new prostheses and plates in carbon-carbon type composite materials are themselves bio-compatible, such prostheses and plates are currently fixed in place by means of special screws made of titanium or of chromium-cobalt-molybdenum alloy with the threads of the screws engaging in the portion of bone that surrounds fixing holes drilled for this purpose into the bone wall.
It might be supposed that metal screws could be replaced by composite screws of the carbon-carbon type in order to make the entire assembly of support and fixing elements bio-compatible, thereby completely eliminating the risk of prosthesis rejection due to metal alloys.
However, the present design of such special long screws includes a highly tapering thread of small root diameter, which makes it impossible to use composite materials for screws of identical or similar size. The small cross-section of such screws could not withstand the torque applied thereto while the screws were being screwed into place.
One of the aims of the present invention is to provide a bio-compatible retention pin whose structure is suitable for implementation with materials that are less strong than metal alloys.
Naturally the threads of prior art fixing screws penetrate into the bone tissue surrounding the fixing holes either by being screwed into previously tapped holes or else by acting as self-tapping screws, in which case the screw itself cuts its own path through the bone tissue, thereby requiring relatively strong bone tissue in the first place.
Another aim of the invention is to provide a retention pin which avoids any need to make notches or grooves or the like in the wall of pin-receiving fixing holes, and to establish contact with forces extending substantially perpendicularly to the axis and to the cylindrical wall of the fixing holes while still setting up an axial retaining force.
Another aim of the invention is to provide a pin whose cylindrical contact face presses, where appropriate, resiliently against the inside wall of the corresponding fixing hole.
The invention also seeks to provide a retention pin which is locked into place only after one of ends of the pin has passed through the fixing hole and has projected out through the other end thereof.